1. Field of the Invention
The invention relates to processing of semiconductor devices and, in particular, to curing of photoresist films employed during such processing.
2. Description of the Prior Art
As is well-known, photoresist films find a variety of applications in semiconductor processing. Typically, photoresist solution is applied to a substrate and is prebaked to drive off solvents, thus leaving a thin film as a residue. A pattern of some sort, for example, a circuitry pattern, is then overlaid over the film, and those portions not covered by the pattern are exposed to electromagnetic radiation, typically in the ultraviolet region of the spectrum, or to a beam of electrons of appropriate energy. During developing of the film employing conventional procedures, the exposed portions (positive resist) or the unexposed portions (negative resist) are removed. In the areas where the photoresist film has been removed, the underlying substrate becomes accessible for further processing. Such processing may involve, for example, the selective removal of an underlying oxide and the deposition of a metal contact. As a final step, the remainder of the photoresist film is removed.
During the course of the foregoing processing, there are drying or curing steps, so-called pre-bake and post-bake steps, which are conventionally carried out in air at about 95.degree. to 120.degree. C. for about 20 to 25 minutes. Such baking consumes a considerable portion of processing time and energy. Further, exposing the entire wafer to temperatures on the order of 120.degree. C. tends to have a deleterious effect on certain finished devices.
Attempts have been made to speed up the baking process. For example, at the 1970 International Hybrid Microelectronics Symposium, Beverly Hills, CA, November 6-18, 1970, a paper was presented by J. Karp entitled "Curing Photoresist Using Microwave Energy". A general purpose microwave oven was disclosed for curing photoresist films deposited over metallized layers on insulating substrates. Drying time for the resist was given as approximately less than 20 seconds for films of about 1 .mu.m thickness. The microwave oven used had a power output of 500 watts (nominal). The process was limited to a maximum time of about 20 seconds, as noted above, since curing cycles ranging between 1 and 5 minutes were found to burn the photoresist and/or destroy portions of underlying metallization. Following exposure of portions of the film and further photolithographic processing, the exposed portions of the cured film were then removed in a chemical etchant.
In contrast to the foregoing publication which discloses use of microwaves to cure photoresist films, Japanese Kokai 10-43079 discloses heating exposed photoresist films deposited on semiconductor layers by microwave radiation to damage the cross-linked structure of the film, which is then completely decomposed by ozone. The photoresist film can thus be easily removed without the use of chemical etchants.